How to mask triangle out of the video strip in VSE?
Use create a Mask (in an UV/Image Editor set to Mask Mode) covering the content you want to keep. You can Ctrl+Click to add Points to create a Mask and Alt+C to Toggle Cyclic so that the first and last point are connected. Use V to Set Handle Types if needed.
Set your Transform Effect Strips Blend type Properties (N) each to Alpha Over. Oh and you'd better mute (H) your underlying Video Strips when you use an Effect Strip on top to reduce unwanted side effects.
Use that Mask going to the Modifiers Properties of your Video Strip in the VSE, click Add Strip Modifier, set it to Mask instead of Strip (Type of input data) and select your Mask created in Step 1.